1. Field of the Invention
The present invention relates to bushings and particularly, though not exclusively, to bushings for use in motor vehicle suspension systems.
2. Background Art
Typically, a bushing is located between a suspension component e.g. one end of a wishbone and the vehicle chassis or body. The bushing incorporates a resilient component for the purposes of cushioning shocks, reducing vibrations transmitted from the vehicle's wheels to the vehicle body and passengers and allowing movement of the vehicle relative to the vehicle body.
One known type of bushing consists of inner and outer metal cylindrical sleeves separated by a piece of rubber. It can be designed to have a linear or non-linear deflection versus force characteristic. Another known type of bushing is the hydraulic bushing, typically including a rubber portion having a plurality of interconnected cavities which contain hydraulic fluid. A hydraulic bushing typically has a non-linear, progressive stiffness whereby the amount it deflects tends towards a constant value with increasingly high loads.
The current automotive trend for large wheels and low profile tires results in increasing suspension loads needing to be withstood by the suspension structure and bushings. One known solution to this problem is to make the bushing bigger, thereby increasing its load capability. However, large rubber bushes have a tendency to induce steering wheel shimmy and large hydraulic bushes tend to degrade impact performance, making the suspension feel harsh. A large bushing also introduces packaging problems in the design of the suspension.
US-A-2003/0137087 describes a series bushing comprising a first inner spring portion incorporating hydraulic fluid cavities and a second outer spring portion formed of microcellular polyurethane. The presence of the polyurethane material permits good isolation of low amplitude, high frequency vibrations but would be too weak to withstand high impact loads, such as a vehicle tire striking the edge of a pothole, for example.